Field of the Invention
The present invention relates to platelet-shaped anticorrosion pigments, to an anticorrosion coating comprising the inventive anticorrosion pigments, to an article coated with the inventive anticorrosion pigments or with the inventive anticorrosion coating, to a process for producing the inventive platelet-shaped anticorrosion pigments, and to the use of the inventive anticorrosion pigments.
Corrosion generally refers to a chemical or electrochemical reaction of a metallic material with components from the environment. Corrosion may lead, for example, to oxidation of an article, for example a motor vehicle body, an aircraft fuselage, a bridge, etc. Corrosion of iron is also called rusting. As well as iron, a multitude of further metals or alloys can also corrode, and therefore be oxidized, for example aluminum or aluminum alloys.
Corrosion causes damage to buildings, motor vehicles, ships, aircraft, etc., which can result in these articles being unable to function.
If, for example, various metals having different electrochemical potential come into contact under moist conditions, for example in the presence of salt water, an electrochemical cell can thus arise. This contact results in oxidation of the base metal.
This effect can be utilized for corrosion protection, by applying a base metal as sacrificial anode to a more noble metal which is to be protected.
Under the action of moisture, the sacrificial anode is dissolved by oxidation and the more noble metal to be protected remains intact.
For example, ship's propellers made from aluminum bronze which come into contact with aggressive salt water are protected by positioning sacrificial anode blocks of zinc on the ship's propeller and along the ship's hull.
Description of Related Art
WO 2008/125610 A1 discloses a coating composition comprising zinc-bismuth alloy particles. The zinc content is at least 95% by weight, preferably at least 98% by weight. The bismuth content is within a range from 0.05% to 0.7% by weight.
WO 2010/043708 A1 discloses a method for electrochemical corrosion protection of a metallic surface, wherein there is a difference in the metal particle concentration and/or the metal particle composition at the interface facing the workpiece to be protected from corrosion and the interface facing away from the workpiece. Metallic particles used may be a multitude of different particles, for example zinc, aluminum, tin, magnesium, nickel, etc., in the form of platelets, lamellae, grains or dust.
KR 2010/023855 A discloses an anticorrosion pigment consisting, for example, of a zinc-magnesium alloy or an aluminum-magnesium alloy. In the pigments composed of the zinc-magnesium alloy, the zinc content is within a range from 90% to 99.5% by mass, and the magnesium content within a range from 10% to 0.5% by mass. Exceptionally preferably, the alloy consists of 98% by mass of zinc and 2% by mass of magnesium.
DE 10 2009 028 667 A1 discloses an anticorrosion pigment having a core-shell structure, wherein the core consists of one or more metal particles. This core has been provided with a shell having hydrophobic groups on its exterior side. The metal particles are selected from the group consisting of magnesium, zinc and aluminum. Alternatively, it is also possible to use metal alloy in the form of metal particles having a main constituent which is one of magnesium, zinc and aluminum. The core having one or more metal particles is spherical in shape.
GB 846,904 discloses a binary zinc-magnesium alloy containing 10% by weight of magnesium. This zinc-magnesium alloy is in the form of bars which are first crushed mechanically and then ground in a ball mill down to a size of 10 μm. The resultant product is in powder form.
DE 10 2007 021 602 A1 discloses zinc-magnesium alloy particles without detailed specification as anticorrosion pigments.
EP 2 246 396 A1 discloses the use of zinc-nickel, zinc-tin, zinc-iron, zinc-aluminum and zinc-aluminum-magnesium alloy as metal powder for use in corrosion protection.
U.S. Pat. No. 2,877,126 discloses the use of a metal powder composed of a binary magnesium-zinc alloy for corrosion protection. The proportion of magnesium therein is within a range between 15% and 30% by weight, and the proportion of zinc within a range from 70% to 85% by weight.
Platelet-shaped zinc pigments and zinc alloy pigments such as zinc-aluminum alloy pigments and zinc-tin alloy pigments are commercially available. They are manufactured and sold, for example, by Eckart Suisse. They are manufactured by grinding zinc powder in ball mills in petroleum spirit and typically stearic acid as lubricant. However, pure ZnMg alloy particles are currently available only in approximately spherical form obtainable from atomization. This is attributed to the fact that these particles are very brittle and are more likely to be crushed than formed in the grinding operation under standard conditions.
US 2004/0191555 A1 discloses anticorrosion pigments based on particulate zinc which may be alloyed with aluminum, tin, magnesium, nickel, cobalt, manganese and mixtures thereof.
EP 2 060 345 A1 discloses zinc alloy particles for use in anticorrosion paints. The zinc alloy particles may contain 0.01% to 30% by weight of magnesium. The aspect ratio of the zinc alloy particles must be between 1 and 1.5. Even an aspect ratio of more than 2 has been found to be disadvantageous.
U.S. Pat. No. 8,114,527 B2 discloses an anticorrosion coating comprising zinc alloy particles which may contain 0.1% to 30% by weight of magnesium. The anticorrosion coating contains the zinc alloy particles in a bimodal particle size distribution. The fine fraction has a particle size diameter of 0.05 to 5 μm and the coarse fraction a particle size diameter of 6 to 100 μm.
In principle, in corrosion protection, the use of platelet-shaped pigments is advantageous since platelet-shaped pigments, unlike pigments having spherical or irregular geometries, have a barrier effect after application to the article to be protected from corrosion.
Pure Zn particles can be shaped into a platelet shape, but the electrochemical potential of pure zinc is too low for such pigments to be usable as sacrificial anode for particular metals or alloys such as aluminum and alloys thereof. Alloy pigments such as ZnMg26 do have sufficient electrochemical potential, but are very brittle and therefore break up extremely easily in the attempt to grind them by standard methods. For example, GB 846,904 discloses that an end product in powder form is obtained by grinding zinc-magnesium alloy particles. In this powder form, the comminuted zinc-magnesium particles are present with spherical and/or irregular geometry. Anticorrosion pigments such as chromates have been found to be highly efficient over many years in applications such as the aerospace sector, but future use appears to be impossible because of their toxicity. Accordingly, there is a need to provide novel anticorrosion pigments which, for example, offer improved properties compared to the known ZnMg particles and/or can serve as a replacement for strontium chromate, for example, in fields of application such as the aerospace sector.